This invention relates to fuel sub-assemblies for liquid metal cooled fast breeder nuclear reactors.
A fuel assembly for a liquid metal cooled fast breeder nuclear reactor comprises a bundle of spaced fuel pins within a tubular wrapper or sleeve through which liquid metal coolant can be flowed in heat exchange with the fuel pins. The wrapper is extended at one end by a tubular neutron shield of massive steel and the other end, hereinafter referred to as the lower end, has a spike extension whereby the sub-assembly can be located by plugging into a support structure and arranged with other sub-assemblies to upstand in side-by-side array to form a fuel assembly. Hitherto, it has been proposed, see for example U.S. Pat. No. 3,383,287, to arrange the fuel assembly of a liquid metal cooled fast breeder nuclear reactor in a plurality of groups of sub-assemblies and for the sub-assemblies of each group to be biassed inwardly towards the centre of the group so that the sub-assemblies of the group lean on each other or on a central support member. Whilst such a system has the advantage of elimination of vibrational movement of the sub-assemblies due to coolant flow it is now recommended that the fuel assembly of a liquid metal cooled fast breeder nuclear reactor should be restrained as a whole from the periphery, that is, the fuel sub-assemblies of the fuel assembly should be centripetally urged together in parallel vertical array. For a fuel assembly of this kind some provision should be made to allow lateral displacement of the wrappers of the sub-assemblies relative to the spike extension in order to relieve stresses throughout the fuel assembly.
Accordingly it is an object of the present invention to provide a construction of fuel sub-assembly which has sufficient flexibility to enable some lateral movement of the wrapper relative to the spike extension.